Rocks and Minerals
Rocks
- Rocks and stones are naturally occurring solids made up of minerals.
- The Earth's crust is made up of rock.
- Rocks have been used by humans for millions of years, from early tools and weapons through to various construction materials.
- There are three different types of rocks based on the way they form, igneous, sedimentary and metamorphic.
- When magma cools and solidifies it forms igneous rock.
- Examples of igneous rocks include granite, basalt, gabbro, obsidian and pumice. More igneous rock facts.
- Sediment deposited over time, often as layers at the bottom of lakes and oceans, forms sedimentary rocks.
- Examples of sedimentary rocks include sandstone, mudstone, flint, greywacke and chalk. More sedimentary rock facts.
- Extreme pressure and heat over time forms metamorphic rocks.
- Examples of metamorphic rocks include marble, quartzite, schist, granulite and slate. More metamorphic rock facts.
- The metamorphic rock marble is formed from the sedimentary rock limestone.
- The metamorphic rock granulite is formed from the igneous rock basalt.
- Minerals are naturally occurring substances formed by geological processes. They are usually solid and feature a crystal structure as well as specific physical properties and chemical composition.
- Physical properties of minerals include crystal structure, hardness (Mohs scale), lustre (how it reflects light) and color, as well as more complicated properties such as streak, fracture, cleavage and density.
- Mohs scale of hardness measures the ability of minerals to scratch each other. Talc is very soft and is listed as 1, diamond is very hard and is measured as 10.
- A mineral is sometimes made up of just one chemical element but more often it is a compound (mixture) of two or more.
- For example, diamond is made up of just carbon, while fluorite is made up of a compound of calcium and fluorine.
- There are over 4000 different types of minerals.
- Only around 30 of these are commonly found in the Earth's crust.
- Examples of minerals include calcite, gypsum, feldspar, pyrite, gold, quartz and diamond.
- The precious gems ruby and sapphire are varieties of the mineral corundum.
The Rock Cycle chart below shows each type of rock connected by arrows. The arrows point from one rock form to a new form that it can become over time and exposure.
The Rock Cycle describes the process the earth uses to recycle rocks. Yes, even rocks are recycled.
There are three basic types of rocks: igneous, metamorphic, and sedimentary. The interesting part of knowing these names is that any one of the three types of rock can be changed into one of the other types.
The names of the rock types refer to the way the rocks are formed. Igneous rocks are formed from fiery molten magma. Metamorphic rocks form under intense heat and pressure. Sedimentary rocks form by weathering.
Igneous Rocks
Igneous rocks are formed of magma, the molten form of the earth’s mantle layer. Igneous rocks can form above ground as lava spewing from volcanoes. But igneous rocks can also form below the surface. Pockets of magma get stuck in layers of the earth. As they get closer and closer to the surface, the magma slowly cools. Granite is an igneous rock that formed from a slow-cooling pocket of magma.
Sedimentary Rocks
Sedimentary rocks form from small weathered particles of other rocks or the weathered shells of sea animals. Wind and rain beating on the faces of exposed rock tend to wear off particles that are blown or washed to a new location. When sea creatures die, the shells settle on the bottom of the ocean. As the sediments pile up, they press together to form Sedimentary rock.
Metamorphic Rocks
Metamorphic Rocks form under intense heat and pressure. Metamorphic rocks start out as igneous rocks, sedimentary rocks or other types of metamorphic rocks, but through heat or pressure, change characteristics such as sheen, tightness of grain and hardness.
Rocks continually change form. What started out as sedimentary rock may change to metamorphic and, with time and weathering, change back to sedimentary.